1. Field of the Invention
This invention relates generally to flat panel displays, and more particularly to flat-panel displays having redundant elements to replace defective circuit elements.
2. Description of the Background Art
As shown in FIG. 1, a typical flat-panel display (e.g., a liquid crystal display for use with a computer) 100 includes a number of pixel cells 110(r,c) arranged in a rectangular array of m rows and n columns, a data bus driver 112 which asserts data signals on data lines 114(c), and an address bus driver 116, which sequentially asserts row-select signals on address lines 118(r). Each pixel cell 110 includes a capacitor 120, a transistor 122, and a pixel electrode 124. Transistor 122 has a gate terminal 126 coupled to address line 118(1), a drain terminal 128 coupled to data line 114(1), and a source terminal 130 coupled to capacitor 120. When address bus driver 116 asserts a high signal on address line 118(1), transistor 122 is turned on, allowing capacitor 120 to be charged or discharged, depending on the signal on data line 114(1), thus storing the data signal. Capacitor 120 is coupled to pixel electrode 124, and asserts the stored signal thereon. Pixel electrode 124 contacts an optically active pixel element (not shown), for example a liquid crystal cell, and the signal applied to the pixel electrode modulates the amount of light passing through the active pixel element.
Flat panel display 100 may be a reflective display or a transmissive display. In the case of a transmissive display, incident light passes through the optically active pixel element, the pixel electrode 124, and the substrate upon which the display is formed. Therefore, in transmissive displays, the electronic elements such as transistor 122 and capacitor 120 must be formed in gaps between the pixel electrodes 124(r,c), so as not to interfere with the light passing through the display. In a reflective display, light is reflected off the pixel electrode, and does not pass through the substrate. Therefore, in a reflective display, the electronic elements may be formed below the pixel electrode. In either case, broken data or address lines or defective electronic components can render one or more of the pixel cells inoperative, thus degrading the image generated by the display.
Because the display output is viewed as a complete image, corrective techniques typically employed in semiconductor memory devices to cope with defective array elements, e.g., the physical relocation of a row or column, are unsuitable for flat panel displays. Many attempts have been made to overcome the problem of defective elements in flat panel displays, but none have been completely successful. For example, U.S. Pat. No. 4,995,703, issued to Kesao Noguchi, describes the division of a single pixel into four separate pixels, each with its own pixel electrode and switching transistor. If a defect renders, for example, two of the four pixels inoperative, the remaining pixels may continue to function, rendering the defect less objectionable. However, this approach suffers from the disadvantage that twice as many data and address lines are required. More importantly, the four separate pixels must be separated by a finite gap, thus reducing the active pixel surface area, and causing a reduction in brightness.
Another technique is described in U.S. Pat. No. 4,680,580, issued to Yukito Kawahara. Kawahara describes a display wherein pixels whose scan or signal lines are broken are serviced by the scan or signal line of a neighboring pixel. While this technique yields a defect that is less objectionable than a row or column that is stuck on or off, it fails to completely compensate for a defective row or column, because the defective row or column is written with data intended for the neighboring row or column, and the original data intended for the defective row or column is lost.
What is needed is a display capable of replacing a defective row or column (e.g., broken row select lines, broken data lines, or defective circuit elements), while insuring that all pixels in the array, including the pixels of the defective rows or columns, are written with their intended data.
A novel display, capable of replacing defective elements, for example broken row select lines, broken data lines, or defective circuit elements, by column and/or row shifting is described. The display generally includes a plurality of pixel cells, each having a pixel electrode in contact with an optically active element (e.g., a liquid crystal element), a primary storage element, and a switch for selectively coupling the pixel electrode with the primary storage element and at least one other storage element. The display may be either transmissive or reflective. In the case of a reflective display, the pixel electrodes are, for example, metallic mirrors, deposited over the switches and storage elements formed on a silicon substrate. The present invention recognizes that redundancy can be provided in the circuit elements, apart from the pixel electrodes and the optically active elements, such that replacement circuit elements can service a pixel electrode in its original position. Thus, no redundancy is necessary for the pixel electrodes or the optically active elements.
In one embodiment the display replaces defective elements by column shifting. A switch, for example a single bit multiplexer, selectively couples a pixel electrode with a primary storage element, for example an SRAM latch, and a storage element of an adjacent column. This embodiment further includes a controller, having a first voltage supply terminal, a second voltage supply terminal, a fuse, and a terminator. The fuse has a first end coupled to the first voltage supply and a second end coupled, via a control line, to the switch. The terminator is intercoupled between the second end of the fuse and the second voltage supply terminal, and operates to maintain a low voltage on the control line when the fuse is opened. Responsive to the low signal on the control line, the switch couples the pixel electrode with the storage element of the adjacent column. A data router, coupled to the control line, redirects data from the primary storage element to the adjacent storage element, responsive to the low signal on the control line.
Typically, any defective elements are identified and replaced during manufacturing. For instance, after the display has been fabricated, but before the display itself is completed, test data is conventionally written to and read from the storage elements of the display. Defects are identified by comparing the written to the read data. This information is then transferred to a computer controlled laser, which vaporizes the appropriate fuses, which are typically made of polysilicon or metal, of the controller, initiating the column shift. Optionally, the replacement of defective elements can occur in the field, by providing, for example, a software programmable controller. Of course other types of programmable elements can be substituted for the fuses.
In a second embodiment, the display replaces defective elements by row shifting. This embodiment includes a pixel electrode, a primary storage element, a second storage element, and a switch. The switch selectively couples the pixel electrode with the primary storage element and the second storage element, disposed in an adjacent row. This embodiment further includes a controller, coupled to the switch via a control line, for initiating a row shift. A row-select router, coupled to the control line, redirects row-select signals from the primary storage element to the second storage element.
In a third embodiment, the display replaces defective elements by column and row shifting. This embodiment includes a pixel electrode, a first switch, for example a two-bit multiplexer, a second switch, for example a two-level multiplexer, a primary storage element, a second storage element, a third storage element, and a fourth storage element. The first switch selectively couples the pixel electrode with the primary storage element, the second storage element disposed in an adjacent column, and the second switch. The second switch selectively couples the first switch with the third and fourth storage elements, disposed in an adjacent row, such that the pixel electrode may be coupled with either the third or fourth storage element, via the first and second switches. This embodiment further includes a column controller coupled to the switches via column control lines, a row controller coupled to the switches via row control lines, a data router coupled to the column control lines, and a row-select router coupled to the row control lines.
Finally, in a fourth embodiment, the display includes a pixel electrode, a first storage element, a second storage element, a first switch, and a second switch. The first switch selectively couples the pixel electrode with the first storage element, and the second switch selectively couples the pixel electrode with the second storage element. Optionally, the switches are CMOS transmission gates, each having a pair of control terminals. This embodiment further includes an activator and a controller. The activator is coupled to the controller and, via a pair of control lines, to the control terminals of the switches. Responsive to a signal from the controller, the activator asserts complimentary (inverse) signals on the pair of control lines. The switches are connected to the control lines in a complimentary fashion, such that both switches are neither on nor off at the same time.